1. Field of the Invention
The present invention generally relates to semiconductors, and more specifically to PMOS technology.
2. Background
A semiconductor device is generally fabricated by performing operations on a semiconductor substrate to provide certain electrical characteristics. These operations can include deposition, diffusion, etching, ion implantation, lithography, and oxidation, to provide some examples.
The semiconductor substrate is typically formed of silicon (Si) and can be either N-type or P-type. An N-type substrate is generally doped with phosphorus atoms (e.g., phosphorus atoms are implanted into, or otherwise added to, the substrate) to provide excess electrons that can be mobilized by applying a voltage to the substrate. A P-type substrate is generally doped with boron atoms to provide excess holes that can be mobilized. Excess electrons or holes are commonly referred to as excess carriers.
A P-type metal oxide semiconductor field effect transistor (PMOSFET), for example, can be fabricated by implanting phosphorus atoms into a P-type substrate to create an N-well. P+ regions are formed in the N-well to provide source and drain regions. The “+” indicates that the region is doped with a higher carrier concentration than a region not designated by a “+”. For instance, the P+ regions typically have a greater number of excess holes than the P-type substrate. An N+ region generally has a greater number of excess electrons than an N-type region.
A gate is located (typically on an oxide layer, called a “gate oxide”) between the source and the drain to control the on/off state of the PMOSFET. The PMOSFET is generally turned on when the voltage applied to the gate goes below a threshold. The threshold voltage of a conventional PMOSFET is negative, and the magnitude of this negative threshold voltage is too great for the PMOSFET to be used in some applications.
What is needed is a low threshold voltage PMOS apparatus and method that addresses one or more of the aforementioned shortcomings of conventional PMOSFET devices and methods.